This invention relates generally to EMI filtered connectors. More specifically, the present invention relates to EMI filtered connectors which utilize one or more internally grounded feedthrough capacitors.
Internally grounded ceramic feedthrough filter capacitors greatly improve the reliability and reduce cost of EMI filters for medical implant terminals. Exemplary internally grounded feedthrough capacitors are shown and described in U.S. Pat. No. 5,905,627 entitled INTERNALLY GROUNDED FEEDTHROUGH FILTER CAPACITOR, the contents of which are incorporated herein.
Ceramic feedthrough capacitors are used in a wide range of electronic circuit applications as EMI filters. Feedthrough capacitors are unique in that they provide effective EMI filtering over a very broad frequency range. For example, this can be from a few Kilohertz to tens of Gigahertz. The mounting or installation of feedthrough capacitors in a typical electronic circuit is always problematic. For one thing, in order to provide proper shielding and attenuation the EMI filter must be installed as a continuous part of the overall EMI shield. This overall EMI shield is usually metallic. Because of the metallic nature of most EMI shields, the installation of a relatively brittle barium titinate-based ceramic capacitor is inherently problematic. This is due to mismatches in thermal coefficient of expansion and resulting mechanical stresses, which can fracture the relatively brittle monolithic ceramic capacitor and lead to either immediate or latent electrical failures. The internally grounded capacitor described in U.S. Pat. No. 5,905,627 was designed to overcome these difficulties.
FIG. 1 illustrates filtered connectors 20a-20l that are typically used in the military, aerospace, medical, telecommunication and other industries. In an EMI filtered connector, such as those typically used in aerospace, military, telecommunications and medical applications, it is very difficult to install the feedthrough capacitor to the connector housing or back shell without causing excessive mechanical stress to the ceramic capacitor. A number of unique mounting schemes are described in the prior art, which are designs that mechanically isolate the feedthrough capacitor while at the same time provide the proper low impedance ground connection and RF shielding properties. This is important because of the mechanical stresses that are induced in a filtered connector. It is problematic to install a relatively brittle ceramic feedthrough capacitor in a filtered connector because of the resulting mismatch in thermal coefficient of expansion of the surrounding materials, and also the significant axial and radial stresses that occur during connector mating.
By definition, connectors come in female and male versions to be mated during cable attach. The EMI filtering is typically done in either the female or the male portion, but usually not both. During the insertion or mating of the connector halves, significant mechanical forces are exerted which can be transmitted to the feedthrough capacitor.
As described in U.S. Pat. No. 5,905,627, the capacitor ground electrode plate is internally attached to one or more lead wires, which can pass all the way through the device or to one or more grounded studs. In the '627 patent, these capacitors were shown uniquely mounted to a variety of implantable medical hermetic terminals such as those used in cardiac pacemakers, implantable defibrillators and the like. By way of example, U.S. Pat. No. 5,905,627 illustrates a rectangular feedthrough capacitor with an internally grounded electrode, which is also shown as FIGS. 2 through 6 herein.
More particularly, an internally grounded feedthrough filter capacitor assembly is generally designated in FIG. 6 by the reference number 22. The feedthrough filter capacitor assembly 22 comprises, generally, at least one conductive terminal pin 24 and a conductive ferrule 26 through which the terminal pin passes in non-conductive relation. An insulator 28 supports each conductive terminal pin 24 from the conductive ferrule 26 in electrically insulated relation, and the assembly of the terminal pins, the conductive ferrule and the insulators comprises a terminal pin sub-assembly 30. The feedthrough filter capacitor assembly 22 further includes a feedthrough filter capacitor 32 that has first and second sets of electrode plates 34 and 36. A first passageway 38 is provided through the feedthrough filter capacitor 32 through which the terminal pin 24 extends in conductive relation with the first set of electrode plates 34. The feedthrough filter capacitor 32 further includes a second passageway 40 into which a ground lead 42 extends. The ground lead 42 is conductively coupled to the second set of electrode plates 36 and the conductive ferrule 26. Typically, the conductive ferrule 26 is conductively mounted to a conductive substrate 44 that may comprise, for example, the housing for an implantable medical device.
The internally grounded feedthrough filter capacitor assembly 22 eliminates the need for external conductive connections between the capacitor and a ground by connecting the internal ground plates to a ground pin, tubelet, or similar ground lead structure. This is a particularly convenient and cost effective approach for certain implantable cardioverter defibrillators (ICDs) that already employ a grounded terminal pin in order to use the titanium housing of the implanted ICD as one of the cardiac electrodes. As there is no external electrical connection, the need for external capacitor metalization around the capacitor perimeter or outside diameter has been eliminated. This not only reduces expensive metallization firing or plating operations, but also eliminates the joining of materials which are not perfectly matched in thermal coefficient of expansion.
The feedthrough filter capacitor 32 comprises a monolithic, ceramic internally grounded bipolar feedthrough filter capacitor having three passageways extending therethrough. The outer two passageways are configured to receive therethrough respective conductive terminal pins 24, and the internal diameter of the first passageways 38 are metallized to form a conductive link between the first sets of electrode plates 34. As is well understood in the art, the first sets of electrode plates 34 are typically silk-screened onto ceramic plates forming the feedthrough filter capacitor 32. These plates 34 are surrounded by an insulative ceramic material that need not metallized on its exterior surfaces.
Similarly, a second set of electrode plates 36 is provided within the feedthrough filter capacitor 32. The inner diameter of the central or second passageway 40 through the feedthrough filter capacitor 32 is also metallized to conductively connect the second set of electrode plates 36 which comprise the ground plane of the feedthrough filter capacitor 32. The second passageway 40 is configured to receive therethrough the ground lead 42 which, in this particular embodiment, comprises a ground pin.
With reference to FIG. 5, the terminal pin subassembly 30 comprises a plate-like conductive ferrule 26 having three apertures therethrough that correspond to the three passageways through the feedthrough filter capacitor 32. The conductive terminal pins 24 are supported through the outer apertures by means of an insulator 28 (which also may be hermetic), and the ground pin 42 is supported within the central aperture by a suitable conductor 46 such as a solder, an electrically conductive thermal setting material or welding/brazing.
The feedthrough filter capacitor 32 is placed adjacent to the non-body fluid side of the conductive ferrule 26 and a conductive attachment is effected between the metallized inner diameter of the first and second passageways 38 and 40 through the feedthrough filter capacitor 32 and the respective terminal pins 24 and ground lead 42. As was the case described above in connection with the attachment of the ground lead 42 to the conductive ferrule 26, the conductive connection 48 between the terminal pins 24 and the ground lead 42 with the feedthrough filter capacitor 32 may be effected by any suitable means such as a solder or an electrically conductive thermal setting material or brazing. The result is the feedthrough filter capacitor assembly 22 illustrated in FIG. 6 which may then be attached to the conductive substrate 44.
EMI filtered connectors are typically manufactured using monolithic ceramic capacitor arrays 50a and 50b. Examples of these multi-hole capacitor arrays are shown in FIG. 7. Planar arrays can vary in the number of feedthrough holes from one all the way up to several hundred in some cases. In the planar arrays 50a and 50b shown in FIG. 7, both the inside diameter of the feedthrough holes 52 and the entire outside perimeter 54 are metallized. The purpose of the metallization is to connect the electrode plates in parallel and to provide a surface for electrical attachment to the capacitor. The metallization usually consists of a fired-on silver loaded glass frit, plating, or the like (sometimes gold terminations are used). The general material used for the dielectric is barium titinate. Accordingly, these devices, when fired, are very brittle (and mechanically weak). In an EMI filtered connector, the brittle ceramic capacitor does not match the thermal coefficient of expansion of the surrounding connector metallic material (such as the connector housing or back shell). Because of this, mechanical stresses are introduced during capacitor installation, mechanical connector mating and during temperature cycling.
FIG. 8 is a cross-sectional view of a typical filtered connector 56 in a π filter configuration. As can be seen the two ceramic discoidal capacitors 58 are directly attached to the inside diameter of the connector. This results in an area of high stress concentration, which can lead to fractures of the monolithic ceramic capacitor. These fractures can result in either immediate or latent electrical failure. A number of manufacturers of filtered connectors have gone to great lengths to mechanically isolate the ceramic feedthrough capacitor. FIG. 9 is an illustration of such a system, which shows spring contact fingers 60, 62 which isolate the capacitors 58 (disposed on either side of an intermediate ferrite inductor 64) mechanically, both for the ground connections to the connector 56 and the connection between the lead wire 66 and the capacitor inside diameter. This allows the capacitors 58 to structurally float thereby making them much less sensitive to damage during connector insertion or during thermal cycling.
FIG. 10 is a connector manufactured by Amphenol utilizing beryllium copper contact resistance clips 68, which provide the ground spring as previously described in FIG. 9. FIG. 10 also illustrates that a beryllium copper EMI grounding spring 70 has been used at the inside diameter contact of the ceramic capacitor. This assembly has been very successful in the industry; however, it is quite complicated and expensive to manufacture. FIG. 10 further illustrates a machine aluminum alloy shell 72, a stainless steel socket hood 74, front removable machine copper alloy contacts 76, a silicone rubber interfacial seal 78, a high temperature dielectric insert 80, a monolithic planar capacitor array 82, sealing and stress isolating elastomeric gaskets 84, a fixed rear nation contact 86, and a ferrite inductor 88.
FIG. 11 illustrates yet another prior art Amphenol connector which has utilized grounding springs 68 and 70 in order to isolate the monolithic ceramic capacitor array from the mechanical stresses due to the connector itself. Components illustrated in FIG. 11 that are equivalent to the components of the connector of FIG. 10 show the same reference number.
In summary, FIGS. 8 through 11 illustrate various methods of installing ceramic capacitor arrays inside of a connector back shell or housing. As can be seen, the capacitors as installed in FIG. 8 are subject to damage caused by both mechanical and thermal stresses. Solutions as indicated in FIGS. 9, 10 and 11, are effective; however, they are expensive, complicated and not very volumetrically efficient.
Accordingly, there is a need for novel filter connectors which utilize the internally grounded feedthrough capacitor as described above in a variety of filtered connector applications. Modification of the connector is needed to adapt it to be compatible with the internally grounded capacitors. Such modifications must provide a low impedance electrical connection that will operate to several gigahertz while at the same time mechanically isolating the ceramic capacitor so that excessive mechanical stresses do not result. The present invention fulfills these needs and provides other related advantages.